Effects of Synthetic Astaxanthin on Growth, Body Color, Antioxidant Capacity and Immune Response of Plectropomus leopardus

Abstract This study investigated the effects of dietary synthetic astaxanthin on growth performance, body color, digestive physiology, antioxidant capacity, and immune responses in Plectropomus leopardus . Four experimental diets containing different levels of synthetic astaxanthin were formulated: T0 (0‰), T1 (1‰), T2 (2‰), and T3 (3‰). A total of 504 juvenile fish (initial body weight: 323.56 ± 14.64 g) were randomly assigned to four treatment groups with three replicates per treatment (42 fish per tank) and fed the experimental diets for 90 days. The results showed that dietary astaxanthin supplementation significantly improved growth performance, as evidenced by increased final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) ( P < 0.05). Astaxanthin supplementation also enhanced body color by increasing chromatophore density and significantly elevating skin redness (a*) and yellowness (b*) values ( P < 0.05). Furthermore, digestive enzyme activities (LPS, AMS, TPA and CPA) were significantly increased in the astaxanthin-fed groups ( P < 0.05), indicating enhanced digestive and nutrient utilization capacities. Dietary astaxanthin also improved physiological health by increasing antioxidant enzyme activities (SOD, CAT and T-AOC), while reducing lipid peroxidation levels ( P < 0.05). In addition, immune-related parameters(ACP, AKP and LZM) were significantly enhanced, suggesting improved innate immune function and resistance to oxidative stress. Among the treatments, fish fed the T2 diet generally exhibited the most favorable responses in growth, pigmentation, antioxidant status, and immune performance. These findings suggest that dietary synthetic astaxanthin promotes growth, body color, digestive efficiency, antioxidant defense, and immune competence in P. leopardus , and that a supplementation level of approximately 2‰ may represent an optimal dietary inclusion level for this species.